A Legionella pneumophila strain (Jena-1) was isolated from a water sample collected from the hot water system of a scientific institution in Jena, Germany. Protein profile, ubiquinone and fatty acid content of the outer membrane were in accordance with those described for other Legionella pneumophila serogroups. DNA extracted from strain Jena-1 gave a positive amplification by using an L. pneumophila (mip)-specific commercially available PCR-kit confirming that the isolate belonged to the species L. pneumophila. Strain Jena-1 reacted with a monoclonal antibody specific for the major outer membrane protein of the species L. pneumophila and another one recognizing a lipopolysaccharide epitope of L. pneumophila serogroups 2-6, 8-10, and 12-15. Cross-absorption studies using absorbed and unabsorbed rabbit antisera to serogroups 1-15 and the newly isolated strain showed that strain Jena-1 cross-reacted mainly with serogroup 4, and to a lesser extent, with serogroups 5, 8, and 10. These cross-reactions could be removed by cross-absorption without significant effects on the homologous titres. It is concluded that strain Jena-1 represents a new serogroup of Legionella pneumophila.